A large number of molecular sieves are known to have use as catalysts in various hydrocarbon conversion reactions such as one or more of reforming, catalytic cracking, isomerization and dewaxing. Typical intermediate pore size molecular sieves of this nature include ZSM-5, silicalite, generally considered to be a high silica to alumina ratio form of ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, SSZ-32, SAPO-11, SAPO-31, SAPO-41, and the like. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for use in dewaxing processes and are described in U.S. Pat. Nos. 3,700,585; 3,894,938; 3,849,290; 3,950,241; 4,032,431; 4,141,859 4,176,050; 4,181,598; 4,222,855; 4,229,282; and 4,247,388 and in British Pat. No. 1,469,345. Other zeolitic catalysts of slightly larger pore size, but still of, for example, 7.1.ANG. or less, are also known to catalyze such reactions. L-zeolite and ZSM-12 are examples of such materials.
Attempts to utilize such catalysts as are discussed above for converting an oil which has a relatively high pour point to an oil which has a relatively low pour point have led to a significant portion of the original oil being hydrocracked to form relatively low molecular weight products which must be separated from the product oil thereby leading to a relatively low yield of the desired product.
High-quality lubricating oils are critical for the operation of modern machinery and automobiles. Unfortunately, the supply of natural crude oils having good lubricating properties is not adequate for present demands. Due to uncertainties in world crude oil supplies, high-quality lubricating oils must be produced from ordinary crude feedstocks and can even be produced from paraffinic synthetic polymers. Numerous processes have been proposed for producing lubricating oils that can be converted into other products by upgrading the ordinary and low-quality stocks.
It is desirable to upgrade a crude fraction otherwise unsuitable for lubricant manufacture into one from which good yields of lube oils can be obtained as well as being desirable to dewax more conventional lube oil stock in high yield. Indeed, it is even at times desirable to reduce waxes in relatively light petroleum fractions such as kerosene/jet fuels. Dewaxing is required when highly paraffinic oils are to be used in products which need to remain mobile at low temperatures, e.g., lubricating oils, heating oils and jet fuels. The higher molecular weight straight chain normal and slightly branched paraffins which are present in oils of this kind are waxes which cause high pour points and high cloud points in the oils. If adequately low pour points are to be obtained, these waxes must be wholly or partly removed. In the past, various solvent removal techniques were used such as propane dewaxing and MEK dewaxing but these techniques are costly and time consuming. Catalytic dewaxing processes are more economical and achieve this end by selectively cracking the longer chain n-paraffins to produce lower molecular weight products, some of which may be removed by distillation.
Because of their selectivity, prior art dewaxing catalysts generally comprise an aluminosilicate zeolite having a pore size which admits the straight chain n-paraffins either alone or with only slightly branched chain paraffins (sometimes referred to herein as waxes), but which excludes more highly branched materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes. Such processes are used to accomplish dewaxing on feeds which contain relatively low amounts of waxes, generally well below 50%, and they operate by selectively cracking the waxes. These processes are not readily adapted for treating high wax content feeds since, due to the large amount of cracking which occurs, such waxes would tend to be cracked to provide very low molecular weight products.
Since dewaxing processes of this kind function by means of cracking reactions, a number of useful products become degraded to lower molecular weight materials. For example, waxy paraffins may be cracked to butane, propane, ethane and methane as may the lighter n-paraffins which do not contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it would be desirable to limit the degree of cracking which takes place during a catalytic dewaxing process.
Although U.S. Pat. Nos. 3,700,585; 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,222,282; 4,247,388 and 4,859,311 teach dewaxing of waxy feeds, the processes disclosed therein do not disclose a process for producing high yields of a lube oil having a very low pour point and high viscosity index from feeds containing anywhere from a low to a very high wax content, i.e., greater than 80% wax, such as slack wax, deoiled wax or synthetic liquid polymers such as low molecular weight polyethylene.
Since processes which remove wax by cracking will give a low yield with very waxy feeds, isomerization processes are preferred. U.S. Pat. No. 4,734,539 discloses a method for isomerizing a naphtha feed using an intermediate pore size zeolite catalyst, such as an H-offretite catalyst. U.S. Pat. No. 4,518,485 discloses a process for dewaxing a hydrocarbon feedstock containing paraffins by a hydrotreating and isomerization process. A method to improve the yield in such processes would be welcome.
U.S. Pat. No. 4,689,138 discloses an isomerization process for reducing the normal paraffin content of a hydrocarbon oil feedstock using a catalyst comprising an intermediate pore size silicoaluminophosphate molecular sieve containing a Group VIII metal component which is occluded in the crystals during growth. Again, a method which would improve the yield would be welcome.
Lube oils may also be prepared from feeds having a high wax content such as slack wax by an isomerization process. In prior art wax isomerization processes, however, either the yield is low and thus the process is uneconomical, or the feed is not completely dewaxed. When the feed is not completely dewaxed it must be recycled to a dewaxing process, e.g., a solvent dewaxer, which limits the throughput and increases cost. U.S. Pat. No. 4,547,283 discloses converting wax to lube. However, the MEK dewaxing following isomerization disclosed therein severely limits pour reduction and thus, very low pour points cannot be achieved. Further, the catalyst disclosed therein is much less selective than the catalysts used in the present invention.
The present invention is directed to overcoming one or more of the problems as set forth above.